It is known to apply bone cement to bone in orthopedic surgical procedures. For example, as part of the installation of an artificial hip joint, the top of the femur is removed and bone cement is packed into the exposed top of the femoral cavity, whereafter the stem of the hip joint prosthesis can be inserted into the top of the femoral cavity to be held in place therein upon curing of the cement, which bonds to the prosthesis stem and to the bone as well.
It has been known to pack bone cement into the femoral cavity by hand.
A major disadvantage of this procedure is that it tends not to apply sufficient pressure to the cement in the cavity, so that the cement tends not to adequately penetrate the pores of the bone within the femoral cavity and thus produces a bond with the femur which is weaker than desired.
Further, this prior procedure is excessively time consuming, which is particularly disadvantageous with two-part bone cements which cure (harden) quickly and so may harden prematurely, i.e. before the surgeon has time to finish packing the femoral cavity.
Further, packing bone cement into the femoral cavity tends to trap air bubbles in the cement packed into the femoral cavity, resulting in unwanted voids in the cement which weaken the bond between the implant stem and femur.
Further, as a result of factors including those above discussed, the quality of the cement bond tends to vary widely from surgeon to surgeon and between subsequent patients of the same surgeon.
In an attempt to overcome some of these disadvantages of hand packing, several companies now offer mechanical bone cement insertion devices which are broadly reminiscent of the common household caulking gun and cartridge. Typically, these prior bone cement injection devices have a pistol-shaped body for releasably supporting a forward extending cartridge containing bone cement and having a ram actuable by a manually movable trigger for pushing bone cement out the front of the cartridge through a suitable nozzle and into the femoral cavity.
In such prior guns the ram is driven by an elongate rod slidable coaxially toward the cartridge forcibly ejecting cement from the forward end of the cartridge. Pulling the trigger typically acts through a pawl engaging rack teeth distributed lengthwise on the rod to advance the ram, or moves a jam washer into jamming engagement with a smooth rod to advance the rod, and hence the ram. In either case, a series of pulls of the trigger results in a series of advancements of the rod and dispensing of corresponding portions of the cement mixture from the front end of the cartridge.
In some prior guns of this kind, structures are provided for adjusting the dispensed quantity of cement mixture, per trigger pull. However, in such prior devices of which I am aware, this is accomplished by relocating a trigger abutting member to different positions on the gun body, to prevent pulling of the trigger beyond any of several selected points along its path of movement, i.e. to prevent pulling the trigger through its full path.
However, this prior adjustment serves merely to change the volume of cement dispensed per trigger pull. It does not, for example, change the ratio of the force applied to the trigger with respect to the pressure applied by the ram to the bone cement within the cartridge.
Presently popular two-part bone cements which must be mixed and injected quickly, having a curing cycle of perhaps 6 to 8 minutes, for passing from a free flowing liquid state (similar to a syrup or watery pancake batter in consistency) to a putty-like consistency and finally to a hard rigid block. Such prior gun/cartridge devices are used by repetitively pulling the gun trigger to inject, over time, the required total amount of cement into the femoral cavity, during which time the mixed cement continues to stiffen. Moreover, filling a femoral cavity with bone cement first calls for injection of a substantial volume of cement to substantially fill the cavity, followed by application of pressure to the exposed end of the cement column to try to press the cement into the pores of the bone surrounding the femoral cavity. Applicant has noted a relation between the two phenomena immediately above stated, namely the tendency of the cement to be more free flowing during initial filling of the femoral cavity and the tendency of the cement to be stiffer at the end of the filling process when it is desired to provide additional pressure on the cement in the cavity.
Accordingly, the objects and purposes of the present invention include provision of an apparatus for injecting bone cement from a cartridge in orthopedic surgery; in which the operator can switch between dispensing a relatively high volume of bone cement at a relatively low pressure, and dispensing a relatively low volume of bone cement at a relatively high pressure without changing the force applied to the trigger or the displacement of the trigger per trigger pull; in which the operator can elect to dispense in the high volume, low pressure mode while the cement is still easily flowable or before the surgical site (for example a femoral cavity) is near to being filled; in which the operator can elect to use the low volume, high pressure mode after the stiffness of the cement increases during curing and/or while, for example, dispensing the last increment of cement into the femoral cavity while pressing same into the pores in the bone surrounding the cavity; in which the apparatus is freely shiftable back and forth between high pressure/low volume and low pressure/high volume modes by means of a simple control actuable by the finger of the user outside the gun; and in which the change in mode involves a change in the driving connection from the trigger to the rod.
Further objects and purposes of the invention will be apparent to persons acquainted with apparatus of this type upon reading the following specification and inspecting the accompanying drawings.
One aspect of the present invention can be defined as follows. Apparatus for injecting bone cement from a cartridge in orthopedic surgery comprises a body, means for supporting a bone cement containing cartridge on the body, trigger means actuable for movement on the body, ram means advanceable on the body in response to actuation of the trigger means for forcing bone cement from the cartridge to the surgical site, and means operatively interposed between the trigger means and ram means and actuable for changing, between a lower force value and a higher force value, the force appliable by the ram means to the cartridge in response to a given actuation of the trigger means.